1. Field of the Invention
The present invention relates to a fixing mechanism for an alternator for a vehicle, which mechanism can be easily mounted to an engine and be fixed thereto so that the alternator may not be resonated in receipt of vibration from the engine in a direction along a shaft thereof.
2. Description of the Prior Art
FIG. 1 is a vertical sectional view of a conventional fixing mechanism for an alternator, and FIG. 2 is an elevational view of the alternator shown in FIG. 1 under the condition where the alternator is mounted to the engine. Referring to FIGS. 1 and 2, each of stators 3 includes a stator core 3a and a stator coil 3b wound around the stator core 3a. A pair of dish-like first and second brackets 1 and 2 are engaged at their respective opening ends with both outer edges of each stator 3, and are fixed by tightening bolts 20.
Thus, the stators 3 are held between the first and second brackets 1 and 2. Bearings 4 and 5 are fixedly engaged with the first and second brackets 1 and 2, respectively, and are aligned to each other.
A shaft 6 is rotatably supported on the bearings 4 and 5. Claw tooth field cores 7 and 8 are located inside the stator 3, and are fixed to the shaft 6. A field coil 9 is interposed between the inner circumferences of the field cores 7 and 8.
A pair of fans 10a and 10b are mounted at both ends of the field cores 7 and 8, respectively, and are rotated with field cores 7 and 8 by the rotation of the shaft 6.
A slip ring 12 is fixed to the shaft 6. Thus, a rotor 11 is constituted of the shaft 6, the field cores 7 and 8, the field coil 9, the fans 10a and 10b and the slip ring 12.
A brush 13a is designed to slidingly contact the slip ring 12. The brush 13a is received and retained in the inside end portion of the second bracket 2, thus forming a current collector 13 for feeding a current.
The field cores 7 and 8 and the field coil 9 are rotated together with the shaft 6 by a pulley 15 which is mounted on the shaft 6 and is driven by the engine. The required exciter current is supplied through the brush 13a and the slip ring 12 to the rotating field coil 9.
As a result, an alternating current is induced in the stator coil 3b, and this current is then converted into a direct current by a rectifier 14.
The first bracket 1 is formed at the upper portion of its outer periphery with an upper flange 1a, and is similarly formed at the lower portion of its outer periphery with a lower flange 1b. On the other hand, the second bracket 2 is formed at the lower position of its outer periphery with a flange 2a.
A mounting bracket 40 of the engine is sandwiched between the lower flange 1b of the first bracket 1 and the flange 2a of the second bracket 2, and the alternator is fixed to the engine by fastening a mounting bolt 17 to a mounting nut 18.
A bushing 19 is engaged in the flange 2a, and it is moved by fastening the mounting bolt 17 to the mounting nut 18, so as to eliminate mounting gaps between the mounting bracket 40 and the lower flange 1b and between the mounting bracket 40 and the flange 2a.
As apparent from FIG. 2, the upper flange 1a is positionally adjustably fastened to one end of an adjusting plate 30 by a mounting bolt 16. The other end of the adjusting plate 30 is fixed to the engine.
The first and second brackets 1 and 2 are fastened to each other by plural bolts 20.
Generally, the level of engine vibration in the vehicle is high in a piston moving direction, that is, in a radial direction of the alternator. Especially in a multiple cylinder engine, as there exists a phase difference in the vertical motion of each piston, vibration along the shaft of the alternator tends to occur, and the vibration level increases with an increase in engine speed.
FIG. 3 shows a schematic illustration of a vibration model of the vibration along the shaft of the conventional alternator for the vehicle as a vibration system of two-degrees of freedom. FIG. 4 shows natural frequencies f.sub.1 and f.sub.1a of the vibration in the vibration system along the shaft of the conventional alternator. In FIG. 4, f.sub.2 and f.sub.2a show natural frequencies along the shaft of the alternator in the present invention. The natural frequencies f.sub.1 and f.sub.1a are obtained from the following expression. ##EQU1##
Referring to FIG. 3, K.sub.H1 stands for a spring constant of a bracket system in the conventional mounting structure (The spring constant includes bracket and flange rigidities, a mounting bracket rigidity and an adjusting plate rigidity), and K.sub.H2 stands for a spring constant of a bracket system in the mounting structure of the alternator according to the present invention which will be hereinafter described (The spring constant includes bracket and mounting leg rigidities and mounting bracket rigidity.).
Further, K.sub.B stands for a spring constant of the bearings (The spring constant is common in the prior art and the present invention.); M.sub.H stands for mass of the bracket system; and M.sub.R stands for mass of the rotor.
The lower natural frequency f.sub.1 along the shaft in the fixing mechanism for the conventional alternator is present in the range of an excitation frequency along the shaft of the engine in association with an increase in engine speed. Therefore, the conventional alternator is sometimes resonated along its shaft, causing trouble such as breakage of the bracket and the winding.
Further, while the mounting bracket 40 is sandwiched between the lower flange 1b and the flange 2a through the bushing 19, and they are fixed to each other by the mounting bolt 17, the first bracket 1 and the second bracket 2 are fixed through the stators 3 to each other only by the bolts 20. Therefore, fixing strength of both the brackets is weak.
To cope with this problem, the prior art structure is designed to increase the thicknesses of the lower flange 1b and the flange 2a along the shaft, the thickness of the upper flange 1a along the shaft, and the thickness of the adjusting plate 30, thereby increasing the spring constant of the bracket system in the mounting structure. Thus, the natural frequency f.sub.1 along the shaft has been intended to be isolated from the range of the excitation frequency of the engine along the shaft of the alternator.
As mentioned above, in order to prevent the resonance along the shaft of the alternator in the prior art structure, the thickness of the bracket along the shaft and the thickness of the flange have been increased to increase the bracket rigidity and the mounting rigidity. However, there remains a problem that this substantial increase in weight of the bracket reduces the resonance to only a limited extent.